1. Field of the Invention
The present invention relates to an image input device and a method of correcting input signals of the same, and more particularly, it relates to an image input device employed in the field of process printing or the like, which is required to read the original in high fidelity and a method of correcting input signals of the said device.
2. Description of the Prior Art
In a conventional image input device for reading the original through a photosensor such as a photo diode array formed by a plurality of photo-electric conversion elements and CCD (charge coupled device), picture signals read by the photosensor are subjected to so-called shading correction to be supplied to an image processing device, so that the original is read in high fidelity. The shading correction is so perfomed as to prevent influences such as dispersion of difference in sensitivity between the respective elements specific to the photosensor, variation of dark currents and irregular illumination originating in an optical or illumination unit. Data required for such shading correction are stored in memory as reference data for correction in advance to reading of the original so that the shading correction is perfomed by adjusting the gain of the picture signals to be read on the basis of the said reference data for correction.
The aforementioned reference data for correction have generally been fetched in the memory through use of the photosensor for reading the original with a white reference panel located on its focal position thereby to perform main scanning for one scanning line.
In such a method, however, foreign matters such as black spots and dust sticking on the main scanning lines of the white reference panel may be so clearly resolved that false information is fetched as the reference data for correction. Further, when the surface of the reference panel is slightly irregular, the direction of reflection or transmission of light may be changed to some extent depending on the position of scanning to change the amount of incident light, and hence, also in this case, false information is fetched as the reference data for correction. The shading correction cannot be precisely perfomed with such false reference data, whereby the orignal cannot be read in high fidelity.